In self oscillating converters, such as boundary conduction mode buck converters, the detection of a lowest voltage in a voltage swing (i.e. the trough a voltage valley) may be required for determining when to turn on a switch in order to minimize hard-switching. Hard-switching results in significant power dissipation into the switch. Switching on at the lowest point of a drain voltage swing of a MOSFET switch reduces this power dissipation and therefore helps to operate at improved efficiency.
By way of enabling a better understanding, the circuit diagram of a conventional boundary conduction-mode buck converter 10 is shown in FIG. 1A. FIG. 1B shows the variation of gate 11 and drain 13 voltage for the MOSFET switch 12 and the variation of output current 15 over the duration of a switching cycle.
In the first part of the cycle (i.e. in the first stroke), the gate voltage 11 is at a HIGH value and the MOSFET switch 12 is on. The current 15 in the inductor rises linearly to a predetermined peak value, which is sensed, and then the switch is turned off and the second part of the cycle (i.e. the second stroke) is entered. In the second stroke, the gate voltage 11 of the MOSFET switch 12 is at a LOW value and the current in the inductor drops linearly until it reaches zero. When the current in the inductor reaches zero, the voltage at the drain 13 of the MOSFET switch 12 swings due to resonance between the drain capacitance and the inductor. When the gate voltage 11 is then reverted back to a HIGH value (and the MOSFET switch 12 is thus switched on as a result) the drain-capacitor-charge into the MOSFET switch 12 is dissipated and the current 15 rises linearly once again.
As mentioned above, switching on the MOSFET switch 12 at the low value of the drain voltage swing (i.e. at the trough of the voltage valley) is known to be preferable so as to reduce power dissipation into the MOSFET switch 12.
In conventional implementations, the voltage valley is detected by sensing one terminal of a capacitor in the integrated circuit (IC), whose other end is connected to the drain. Such a known approach to detecting the voltage valley is shown in FIG. 2. This requires (a) the drain node to be accessible from the IC, and (2) high voltage capacitor inside the IC.
For controller-only ICs this is not feasible because the drain node is not accessible. Further, it would require either an extra pin with an internal high-voltage (HV) cap (which then necessitates HV components in the IC), or an extra pin with an external HV cap. This results in increased cost and reduced accuracy.
A known alternative approach to detecting a voltage valley is to use a capacitance from the MOSFET drain to a sense pin of the controller. However, this has the disadvantage of requiring an additional HV capacitor and HV resistor.